Significant progress was made on this project during this reporting period. Several structure-activity studies were reported. One examined the DAT/SERT selectivity of flexible GBR12909 analogs modeled using 3D-QSAR methods. A second reported the design and synthesis of 2- and 3-substituted-3-phenylpropyl analogs of 1-2-bis(4-fluorophenyl)methoxyethyl-4-(3-phenylpropyl)piperazine and 1-2-(diphenylmethoxy)ethyl-4-(3-phenylpropyl)piperazine: role of amino, fluoro, hydroxyl, methoxyl, methyl, methylene, and oxo substituents on affinity for the dopamine and serotonin transporters. We also made progress in identifying novel partial inhibitors of amphetamine-induced dopamine release. In particular, we reported: "Previous studies identified partial inhibitors and allosteric modulators of 5-HT (5-amino-3-(3,4-dichlorophenyl)-1,2-dihydropyrido3,4-bpyrazin-7-ylcarbamic acid ethyl ester SoRI-6238, 4-(2-bis(4-fluorophenyl)methoxyethyl)-1-(2-trifluoromethyl-benzyl)-piperidine TB-1-099) and dopamine transporters N-(Diphenylmethyl)-2-phenyl-4-quinazolinamine, SoRI-9804). We report here the identification of three novel allosteric modulators of the dopamine transporter N-(2,2-Diphenylethyl)-2-phenyl-4-quinazolinamine SoRI-20040, N-(3,3-Diphenylpropyl)-2-phenyl-4-quinazolinamine SoRI-20041, 4-Amino-6-(diphenylmethyl)amino-5-nitro-2-pyridinylcarbamic acid ethyl ester, SoRI-2827). Membranes were prepared from HEK cells expressing the cloned human dopamine (hDAT) transporter. 125IRTI-55 binding and other assays followed published procedures. SoRI-20040, SoRI-20041 and SoRI-2827 partially inhibited 125IRTI-55 binding with EC50 values ranging from 1.4 M to 3 M and EMAX values decreasing as the 125IRTI-55 concentrations increased. All three compounds decreased the 125IRTI-55 Bmax and increased the apparent Kd in a manner well described by a sigmoid dose-response curve. In dissociation rate experiments, SoRI-20040 (10 M) and SoRI-20041 (10 M), but not SoRI-2827 (10 M), slowed the dissociation of 125IRTI-55 from hDAT by 30%. Using rat brain synaptosomes, all three agents partially inhibited 3Hdopamine uptake with EC50 values ranging from 1.8 M to 3.1 M and decreased the VMAX value in a dose-dependent manner. SoRI-9804 and SoRI-20040 partially inhibited amphetamine-induced DAT-mediated release of 3HMPP+ from rat caudate synaptosomes in a dose-dependent manner. Viewed collectively, we report several compounds that allosterically modulate hDAT binding and function, and identify novel partial inhibitors of amphetamine-induced dopamine release."